Nanotech st5709s1208 and other steppers maximum speed

Hi All,

I am using the Nanotech ST5709S1208 (0.9 degree, 400 steps for a revolution) motor in bipolar configuration to rotate a light mirror mounted on its shaft. I have constructed a driver circuit using a half bridge circuit and a PIC16 microcontroller. I find that the maximum I can drive this motor is about 1.25 - 1.4 revolutions per second without the mirror loosing step. I have tried microstepping the motor with half/8/16 and 32 steps and though I can increase the stepping frequency as the number of microsteps increases, the speed of rotation of the motor is still about 1.25 rps. This is a bit too slow for my optical scanning application and I have a feeling that I may be doing something else wrong since I am new to stepper motors. To control the motor I use a chopping waveform of 20kHz and a linear velocity profile to attain max speed

I would like to know what any of your experience is regarding the maximum speed of this motor in particular and steppers in general and why do you think I might be getting such a low speed in spite of the fact that the torque I require is quite low (load = small, light mirror) ?

Thanks in advance!
Govind

Edited 3 time(s). Last edit at 08/05/2009 05:47AM by govind.

The things that limit the speed are the torque required to accelerate the inertia of the rotor and the mirror, and the inductance of the coils.

To go faster you will probably need to have a few steps of accelaration to get started, and deceleration to stop.

Once you are using acceleration you will find the inductance of the motors limits the max speed. It takes time for the current to build up in the coils after they are switched on. To get higher speeds wire the motor in bipolar parallel and use a chopper drive. Then if that is not fast enough you will need to increase the supply voltage to something like 30V.

Steppers are never going to get very fast, and this one is quite a slow one due to its small step angle and relatively high inductance. Also the rotor inertia is probably significant. If your mirror is small, a smaller motor might be faster.

The absolute maximum speed you are going to be able to get with a small angle stepper will be about 10000pps, so about 25 rps.

A much faster way to drive a mirror is to attach it to something like the head servo of a hard disk if it does not have to do a full rev.

Edited 1 time(s). Last edit at 08/05/2009 06:06AM by nophead.

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[www.hydraraptor.blogspot.com]

Hi nophead,

Thanks a lot for your reply.

I already have a constant velocity profile to slowly accelerate the motor up to the maximum speed, then slew at max speed and decelerate after that before reversing direction. So I think this is not the issue.

You are probably right about the inductance. But I cannot immediately try bipolar parallel (i'm using series) because my in-house driver is not capable of that much current. however, I will try this and your suggestion is very much appreciated because i had never really looked closely at the difference between the types of windings.

When you say 30V, how do I achieve this without damaging my motor which is rated to 6V. Is it by using a high frequency (say 20k) PWM chopping waveform of the appropriate duty cycle ((6/30)*100)?

1208's data sheet:
[en.nanotec.com]

I shall explore other motors too...

Thanks!
Govind

The inductance is 1/4 when wired in parallel.

Yes the current is limited by chopping which is effectively PWM. A chopper circuit turns on until a comparator signals that the target current has been reached, at which point is swicthes off for a while and then repeats. That means the current builds up as fast as possible and then hovers around the set value.

Open loop PWM will not give the same effect, especially when you take the motors back EMF into consideration.

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[www.hydraraptor.blogspot.com]

Greetings all,

One way to increase max. step rate is to increase the voltage used.
A higher voltage rail gives a faster rate of current increase (for a given inductance.) However, first check what your drive circuitry can handle (though it's probably substantially above the +12v that the current reprap designs use.)

HTH,

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Larry Pfeffer,

My blog about building repstrap Cerberus:
[repstrap-cerberus.blogspot.com]

... i had some trouble getting high rpms with the nanotec SMC11-microstepping driver ( [en.nanotec.com] ) and a standard nema-23 stepper with 1.8°/200 spr.

With 12 Volts i only managed to run the stepper with max 6kHz at 1/8 microstep, what's something like 3.75 rpm.

With 30 Volts and the same parameters it was running smooth until 20kHz or 12.5 rpm.

It's highly dependant of the maximum voltage and the accelerating/deccelerating parameters ...

Viktor

Alright, thank you folks for sharing your knowledge, I think the main issues effecting stepper speed are clear to me now. Didn't get this information from some other forums :-)

Govind